Add a Lazy type as alternative to std::sync::Lazy

[phil-opp]

The implementation is copied from the SyncLazy implementation in the standard library.

Resolves #88.

[zesterer]

Thanks very much for this!

[Ericson2314]

I got a lot of déjà vu before I remembered rust-lang-nursery/lazy-static.rs@528ea62#diff-7b86b47fe3d41ce27a46ac8817b97d54 was in that crate not this one!

Seeing #88 I see putting one here is intention and FWIW I agree with that :). I guess someone just needs to go make lazy-static not implement it's own Lazy with or without spin_no_static.

[zesterer]

@Ericson2314 The difference with lazy_static's version seems to be that the function isn't a parameter on the type. That said, I don't see why they can't just use Once directly, it seems to have basically the same API.